We propose to analyze cellular factors in the mouse which control the replication of exogenous murine leukemia virus and the infection of endogeneous MuLV, and to identify the chromosomal locations of endogenous ecotropic and xenotropic viruses. Mouse-Chinese hamster hybrids which segregate murine chromosomes will be produced by somatic cell hyridization and microcell fusion. This process confers susceptibility to infection by exogenous Gross virus WN 1802 B to the hamster cells. Murine chromosome(s) which control this function, which is probably a receptor, will be identified by Hoechst 33258 and Giemsa banding techniques and by isozyme analysis. An "anti-receptor" antibody will be prepared by injecting hamsters with mouse-hamster hybrid cells which retain only a few mouse chromosomes, but can still support exogenous MuLV replication. Bone marrow derived lymphocytes (but not thymus derived lymphocytes) can be infected in vitro by WN 1802 B and Friend leukemia virus. Experiments will be carried out to determine how universal a phenomenon B cell susceptibility is, in regard to other leukemia viruses, and the nature of this susceptibility will be analyzed with the "anti receptor" antibody. We will attempt to overcome the restriction in T cells by introduction of B cell material. In experiments designed to identify the chromosomal location for endogenous leukemia virus, single stranded DNA copies of MuLV will be made and Cot analysis of the (3H) probes will be performed with DNA from a series of mouse-Chinese hamster hybrids which return only a few murine chromosomes. Such cells will also be treated with agents (BudR, cycloheximide) to induce endogenous MuLV. Ecotropic and xenotropic virus induction (assayed by XC test, virion associated DNA polymerase after passage in permissive cells) will be correlated with the presence of individual murine chromosomes.